Modern home TV appliances, movie theaters, and presentation apparatuses often use digital projection systems for the purposes of video or static image demonstration. The conventional projection system comprises a projector and a projection screen. One of the most promising solutions for digital projection employs a liquid crystal display (LCD) as the image-forming device. The LCD forms an image as an array of pixels by selectively modulating the polarization state of incident light for each pixel. High-resolution large-area LCDs can be fabricated more readily than the analogous devices of other types. In addition, small thickness and low weight define the general portability and mobility of projection systems. The LCD-based projection systems are often simpler in manufacturing and practical use, especially when large-area image or video demonstration is required.
Conventional projection screens typically include fine transparent or translucent porous particles embedded in a transparent medium and a reflective material located behind the particles. The projection screens reflect substantially all the incident light, that is, they reflect ambient light as well as light from the imaging source. Since a part of the ambient light is reflected toward the viewers, the image contrast and the apparent brightness of the image is often reduced, particularly in areas where the ambient light intensity is relatively high.
To enhance the brightness of the reflected image, some projection screens include retroreflective elements such as glass beads, which are capable of reflecting the ambient light back in the direction from which it was incident onto the screen. The introduction of retroreflective elements, however, narrows the range of angles over which the image can be viewed because the imaging light is also retroreflected. Moreover, if the source of ambient light is aligned with the viewers, the ambient light can also be retroreflected toward the viewers along with the imaging light.
The brightness of images produced by liquid crystal projectors, in particular, can be relatively low because light of only one polarization state is projected onto the screen due to the nature of a liquid crystal display used to form the image. If the projection screen reflects ambient light at a low brightness of the projected, the image contrast can be significantly reduced. As a result, liquid crystal projectors are used primarily in areas with low levels of ambient light, such as rooms in which the windows are shuttered with curtains and in which artificial lighting is dimmed, to limit the contrast reducing effects of the ambient light. This may be undesirable, however, because it hinders the ability of viewers in the room to consult written materials, take notes, etc. during presentations.
Attempts to solve the brightness and contrast problems associated with liquid crystal projectors have included the use of absorbing polarizers in combination with reflective materials. By incorporating absorbing polarizers in the screens, about one-half of the ambient light can be absorbed in the projection screen rather than reflected as in conventional screens that do not use absorptive polarizing materials.
The absorptive polarizing materials used in the projection screens provide for the preferential transmission of light with the first polarization state and block the light with the second polarization state. The transmitted light is then reflected back from the reflective material and retransmitted through the absorptive polarizing material. Therefore, the liquid crystal projectors use light of only one polarization state to form images—that preferentially reflected by the projection screen. Ambient light, however, typically includes light having both polarization states and, therefore, a significant portion of the ambient light incident on the projection screen is absorbed rather than reflected. As a result, the contrast and apparent brightness of the images formed by the liquid crystal projectors on projection screens employing absorptive polarizing materials can be improved as compared to conventional projection screens reflecting light of both polarization states.
While the ideal absorptive polarizing material transmits all incident light having the first polarization state and absorb all incident light in the second polarization state, real absorptive polarizing materials absorb at least some of the incident light having the first polarization state along with the light in the second polarization state. As a result, some of the imaging light is absorbed rather than reflected, thereby reducing image contrast and brightness. Moreover, in projection screens using absorptive polarizing materials, this material is located in front of the reflector. In this arrangement, the incident imaging light having the preferentially transmitted first polarization state must pass through the absorptive material two times before reaching the viewer. In each passage, the absorptive polarizing material can absorb a significant portion of the light with the first polarization state, thereby reducing image brightness.
Besides the above problems, projection screens with absorbing polarizers that include other elements such as diffusing materials may also suffer from reduced image brightness and/or contrast if those additional elements cause some of the image light to change polarization states. A portion of the imaging light whose polarization changes to the state absorbed by the absorptive polarizing material will not reach the viewer. The result is reduced image brightness and contrast.
Projection systems usually require a large viewing angle characteristic of the projections screen. This requirement is especially desirable for the projections screens working in large rooms or in the outdoor environment. For the convenience of the projection screen viewers, the greater viewing angle is especially important in the horizontal plane.
The common drawback of the conventional polarizing projection screens made of iodine-containing organic polarizers is a small viewing angle. This drawback is due to a rodlike shape of molecules of the iodine-containing organic polarizers. The light polarized along a single direction coinciding with the axis of rod-shaped molecules is absorbed. Any deviation of the light polarization from said direction leads to a sharp decrease of the polarized light absorption.